What advice would you give to someone feeling discouraged or doubtful about their entrepreneurial journey?

I maybe too blunt and not fluffy enough.  A good reminder on why you started the business.  If the business owner is feeling discouraged or doubtful and they cannot change their mindset, they should become an employee. As leaders, we need to be comfortable putting ourselves into uncomfortable situations. Nothing wrong with doubting a decision but if it was made and it turns out wrong, you learn from it and move on.

Consistency over intensity, most people fail because they don't see immediate results from their efforts and just give up.

I found that goal setting and breaking down those goals into individual tasks made it easier to see the path to growth. If 5 big goals for the year have 10 tasks each to achieve them, that's enough for 1 task each week for the year. If you do all that work and still fail, you can honestly say that you did at least 50 things to make it work. That at least helps to address the operational needs of the company. 

In terms of your personal feelings as an entrepreneur, just know that you aren't alone. Everyone who has built a business from scratch have these problems. I'll bet cash is uncomfortably tight? You have a couple clients who are unreasonable but you can't afford to fire them? Your staff aren't nailing it? How about not having enough hours in the day? Beeeeeen there. We all have.

The burnout, the anxiety and the uncertainty will clear up if you're willing to keep pushing forward. That struggle you're going through today is extremely important, necessary even. When you overcome today's big nightmare, (and you will one way or another) you'll come out of it more trained and skillful for nightmares like that. Knock out enough nightmares and guess what, nothing is a nightmare anymore. You'll be less and less vulnerable, or maybe just vulnerable to tackle bigger and scarier issues. There's no pathway to entreprenuerial success that isn't paved with the hard won experience. 

Somewhere along the way, a friend or family member will point out how you kept your cool in a seriously tough situation, and you'll realize that it wasn't tough this time, and you'll remember when it was the toughest thing you could imagine. Riding out entrepreneurship will change who you are.

You just have to start. 
Will it be sacry? Definitely!!
Will it be worth it? Totally!!! 
Will it be easy? **bleep** No! 

Always keep learning, because there will be someone out there that has the newest, shiny thing that will challenge you. But if you dig in and go for it and you know what you are doing, then you have nothing to fear.

There are highs and lows just keep at it and never get discouraged when you have your lows because if you keep on that path it will turn into a high!

Having a bigger purpose or why has always helped me find my true north when the journey is tough

Trust yourself, keep learning, and keep pushing forward. Keep learning new ways to do what you need to do. 

Trust the process. Lulls can happen, and you can always learn something from them. Motivation is intrinsic so revisit your "why" and tweak it if it has evolved.